In the field of microelectronics, circuits are manufactured by means of superimposed and differently structured layers of different materials. In order to achieve the desired planarity, processing has to take place subsequent to each coating of a semiconductor wafer with a layer, for example an oxide layer, a tungsten layer or other metal layers, otherwise problems will occur, for example within lithographic processes, namely in the form of focus faults due to a low depth of sharpness of the UV stepper or in the form of fissures of the conductor paths.
A process used in the current semiconductor industry and designed for the planarisation exploits the so-called CMP process consisting in a chemical mechanical processing by means of a fluid (slurry), the chemically reactive part of the slurry being bound to transform the material into a polishable condition. The slurry contains an abrasive in form of colloidal abrasive particles.
A device for the chemical mechanical polishing of surfaces has been published in DE 197 19 503 A1. It comprised two polishing units with height adjustable vacuum carriers for one semiconductor wafer each which can be rotated about a vertical axis by a driving motor. The polishing units are guided independently from each other along two parallel guides extending approximately horizontally. Polishing tables driven in rotation are arranged below the guides and cooperate with the polishing units. At one end of the guides, at least one device is provided for the semiconductor wafers. Furthermore, loading and unloading devices for the semiconductor wafers are provided on the opposite side of the guides, towards the polishing units can be aligned and which can be reached by a transfer device. In most of the cases, the delivery and take over takes place by a robot.
During the transport of the wafers and the polishing process described above, the wafers are held by a chuck or a so-called carrier the task of which is to generate a homogenous pressure profile or different pressure profiles onto the rear side of the workpiece, the so-called loaded side, i.e. the side provided with the circuits, being oriented towards the polishing table. Usually, the carrier is held and moved by an actuation device turning the carrier around a vertical axis on the one hand and on the other hand moving it in a linear, vertical and horizontal direction.
In most cases, a wafer is held at the carrier by vacuum. First of all, the wafer must be brought into contact with the supporting or engagement surface of the carrier before the vacuum can transfer its holding force to the wafer. During the polishing process, the circular workpiece, for example the wafer, is stabilized by the outer rim of the carrier protruding beyond the supporting surface of the carrier and having an inner diameter which is slightly larger than the outer diameter of the wafer. In order to achieve high quality polishing results on the workpiece, especially in the radially marginal portion of the workpiece, it is absolutely necessary to dimension the protruding edge formed by a retaining ring in such a way that the circular space between the workpiece and the ring is minimized and uniform along the circumference.